ProjectExplorer: Fix memory leaks in tree models

Fix widgets being leaked by the ToolChainOptionsPage.
This page created widgets and stored them in the tree node
and never deleted them.

The fix is to put all the widgets into one QStackedWidget, so
that this will clean up once the page is destroyed.

Change-Id: Ic02824a4c52771d8962dc594176077c2e139fb84
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/qnx/qnxtoolchain.cpp

@@ -118,9 +118,9 @@ QString QnxToolChain::typeDisplayName() const
     return QnxToolChainFactory::tr("QCC");
 }
 
-ToolChainConfigWidget *QnxToolChain::configurationWidget()
+std::unique_ptr<ToolChainConfigWidget> QnxToolChain::createConfigurationWidget()
 {
-    return new QnxToolChainConfigWidget(this);
+    return std::make_unique<QnxToolChainConfigWidget>(this);
 }
 
 void QnxToolChain::addToEnvironment(Environment &env) const